The present invention relates to the separation of solids from liquids by filtration through granular media and, more particularly, to an improvement in apparatus for granular media filters of the type having a tank and a bed of granular filter media in the tank through which liquids having suspended or dissolved solids therein flow for removing the solids to clean the liquid.
Granular media filters are known for removing solids from liquids. Such filters have long been utilized for potable water treatment and have been introduced to wastewater treatment plants, where the solids may comprise microbial flocs, coagulant residues, and a variety of other relatively unpredictable substances. Such filters normally include a tank for receiving flowing liquid containing suspended solids, a bed of granular media supported within the tank, means for removing liquid which has passed through the media (filtrate), and means for periodically washing the granular media to remove particulates collected therein during filtration. Because the washing step is usually accomplished by passing liquid through the bed in a direction opposite to the flow direction for filtration, the washing step is usually referred to as backwashing.
In one well-known granular media filter construction, filtration is accomplished in the downward direction so that filtrate is removed from beneath the media. Such "downflow" filters may be capable of either semi-continuous or continuous operation. In this latter operation, partitioning walls are fixedly mounted to form a plurality of individual cells within the granular media bed so that backwashing can be accomplished in one of the cells while filtration proceeds in the other cells. Such filtering machines are shown in U.S. Pat. Nos. 3,239,061 and 4,151,265.
In "semi-continuous" granular media filters, the entire granular bed is used simultaneously for filtration until the bed collects solids to the extent that its resistance to flow adversely affects the rate of operation of the machine, or the effectiveness of removing solids. Then the machine is removed from filtration service and the entire bed is cleaned as a unit.
Granular media filters may also be of the type used to remove dissolved solids from liquid. Such filters typically use filter media of activated carbon, which physically adsorbs dissolved solids from the liquid. Over time, the activated carbon may adsorb so much dissolved materials as to become saturated or spent. In this event, the spent filter media must be removed from the filter, and new or regenerated media delivered to the filter. Similarly, in granular media filters of the above-described type used to remove suspended solids, the media may after long usage be rendered unsuitable for further filtering, even if backwashed. This media must then be removed, and new media or media cleaned by methods other than backwashing must be delivered to the filter. With either type of filter, the filtration operation of the filter must be terminated while the filter media is being replaced.
A low-head filter apparatus is shown in U.S. Pat. No. 4,624,783. This filter employs a pair of rails, 39a and b, mounted to the top of the filter which support a bridge 31, which in turn supports a partitioning chamber 41. The bridge translates along the rails while the partitioning chamber 41 can be selectively raised or lowered. Partitioning chamber 41 cuts through a section of the media 23, in effect isolating that section, while the balance of the media 23 is available for filtration. With the partitioning chamber 41 lowered through the media 23, that section of the media within the chamber could be backwashed or the media within that partitioning chamber 41 could be completely removed and replaced. A series of valves 57 are included in the partitioning chamber 41 to facilitate venting of entrapped vapors and allow the chamber 41 to move downwardly into the media 23. While this design proved advantageous, it was costly to construct and on occasion presented certain difficulties in obtaining an adequate seal between the underdrain support means 19 and partitioning chamber 41. Occasionally the media 23 became so hard as to impede the downward travel of hood 41 and prevent uniform contact between hood 41 and underdrain support means 19.
Subsequent modifications were made to install a plurality of upwardly extending partition plates from underdrain support means 19. The hood 41 was still used, but instead of having to be lowered down to underdrain support means 19, the hood would be lowered between a pair of upwardly extending partitions. In this design, hood 41 did not have to be brought all the way down to underdrain support means 19. There would be a clearance fit between the hood 41 and the partitions, and what sealing was obtained was achieved when the hood 41 was made to descend onto the partitions to attempt to isolate a given compartment. Some designs incorporated a rigid neoprene member to assist in the compartment isolation. A vacuum source was then applied to partitioning chamber 41 which, depending upon its position in the media, would induce clean water from the filtrate receiving chamber 21 up through hood 41 for backwashing. Other means were provided with the hood to actually remove the filter media through the use of devices such as an eductor. U.S. Pat. No. 4,624,783 also disclosed retractable and movable means with the chamber 41 for the purpose of using a compressible fluid to assist prior to the actual water backwash step. The mechanism shown included a series of probes or lances carried on a movable manifold in communication with a source of pressurized air or liquid wherein the probes 81 could be inserted into the bed.
U.S. Pat. No. 3,984,326, entitled "Gravity Filtering Equipment," showing Mr. Henry Bendel as the inventor, is for gravity filtering equipment comprising a basin partitioned into a plurality of compartments, each having a permeable bottom covered by a filtering bed and a device for flushing such filtering beds. This device comprises a bell adaptable fluid-tightly onto the upper edges of each compartment in turn and provided with a pump for sucking up filtered water, from a collecting chamber provided underneath the permeable bottom of each such compartment, through its filtering bed.
While all the embodiments discussed above proved to be serviceable, the designs still allowed for improvements to be made in the overall cost of construction of such filters, as well as in obtaining greater efficiencies in the sealing isolation of individual compartments being serviced within the filter. The apparatus of the present invention addresses these areas in providing improved sealing techniques for the compartments within the filter, a drive mechanism for the cleaning apparatus which results in greater economies in construction and operation, and an improved technique of lifting and scouring the filter media with a gas to dislodge collected impurities prior to backwashing by fluidizing with water to carry away such impurities.